1. The Field of the Invention
The present invention relates to systems and methods that integrate energy storage and cryogenic carbon capture. More specifically, the invention provides means to store and manage energy generated by power plants or other grid-connected sources and use the stored energy in cryogenic processes to capture carbon dioxide and other condensable vapors from light gases.
2. The Related Technology
The separation of carbon dioxide from other light gases or liquids such as flue gas, nitrogen or methane is important for achieving carbon dioxide sequestration. Process gas (flue gas) from a conventional power station typically includes from about 4% (vol.) to about 16% (vol.) carbon dioxide (CO2) and raw natural gas from a well can include large amounts of CO2. This process technology addresses both these and similar processes containing a condensable, desublimating vapor. This CO2 contributes to greenhouse effect and global warming. Therefore, there is a clear need for efficient methods of capturing CO2 from process gases to produce a concentrated stream of CO2 suitable for transport to a safe storage site or to a further application. Additionally, CO2 in natural gas represents an inert component that is expensive to transport and dilutes the effectiveness of the natural gas. These and similar streams containing a desublimating vapor are addressed by these methods and processes.
CO2 has been captured from gas streams by several technologies, the most common of which include: absorption, where CO2 is selectively absorbed into liquid solvents; oxyfiring, where oxygen is separated from air prior to combustion, producing a substantially pure CO2 effluent; membranes, where CO2 is separated by semi-permeable plastic or ceramic membranes; adsorption, where CO2 is separated by adsorption on the surfaces of specially designed solid particles; chemical looping, where carbon oxidation and oxygen consumption are physically separated by a recycled intermediate, typically metal oxide; and low temperature/high pressure processes, where the separation is achieved by condensing the CO2.
In the past, the most economical technique to capture CO2 from a process gas has been to scrub the process gas with an amine solution to absorb the CO2. This technology has been used commercially for small-scale processes and for specialty processes. For instance, Flour developed a process, called Econamine FG+, for doing so. However, it has not been demonstrated in utility-scale power plants. In all cases, the projected reduction in process efficiency and increase in process costs are high (25-30% and 80%, respectively, according to DOE estimates for power stations).
Another type of process is the oxy-combustion system, which uses oxygen, usually produced in an air separation unit (ASU), instead of air, for the combustion of the primary fuel. The oxygen is often mixed with an inert gas, such as recirculated process gas, to keep the combustion temperature at a suitable level. Oxy-combustion processes produce process gas having CO2, water and O2 as its main constituents; the CO2 concentration being typically greater than about 70% by volume. Treatment of the process gas is often needed to remove air pollutants and non-condensed gases (such as nitrogen) from the process gas before the CO2 is sent to storage.
Cryogenic carbon capture (CCC) processes are emerging new methods for separating CO2 from other gases by condensing CO2. Conventional refrigeration processes are not energy efficient because the processed gases are cooled to a very low temperature, consuming substantial amount of energy in cooling and compressing.
Although Carbon capture and storage (CCS) is highly desirable for controlling carbon output, the energy cost of CCS is high. Capturing and compressing CO2 may increase the fuel needs of a coal-fired CCS plant by 25%-40%. These and other system costs increase the cost of the energy produced by 21-91% for purpose built plants. Applying CCS technology to existing plants would be more expensive if substantial plant modifications or replacement in addition to the CCS technology are required, as is generally the case. Thus, there is a pressing need for energy efficient systems for carbon capturing.
In addition to limiting carbon dioxide emission, the energy industry faces another general challenge: managing energy supply and demand in response to constant fluctuation of energy usage and generation.
Consumption of energy fluctuates with time of the day, week, and season. Many power generation sources cannot practically or economically change generation load according to fluctuation of energy demand. For instance, it would be impractical to idle a nuclear reactor during off-peak hours of the day and re-engage it during peak hours. Coal boilers and all other boiler-style heat engines are similar. As a result, there is great interest in storing energy in excess of demand during non-peak hours and supplying it during peak demand, lest the energy would be wasted.
Moreover, some power generators such as wind turbines and solar farms create intermittent power. The intermittency of such power compromises its effective use since it cannot be accurately anticipated in time or amount and many of the other systems cannot respond to load changes as rapid as wind and solar power changes. This, too, creates a great interest in storing large amounts of grid energy.
Various categories of methods for storing energy include: chemical, biological, electrochemical, electrical, mechanical and thermal. Pumped hydro-storage is among the most reliable and efficient mechanical methods for storing energy, in which electricity is used to pump water into reservoirs during off-peak demand. When demand peaks, the water drives generator turbines. Hydrogen is proposed as a chemical method, where electricity is used to split water molecules to produce hydrogen, which is then burned or reacted in a fuel cell to generate electricity when needed. Compressed air energy storage (CAES) is another mechanical method for storing energy, which uses off-peak power to compress or partially compress air ultimately used in a gas turbine to generate energy.
These methods of energy storage may not be available or economically viable for a specific power plant. For example, pumped storage requires both water and mountains, both of which are highly regulated and often difficult to implement. Hydrogen storage is both very expensive and inefficient and hydrogen storage requires extraordinary materials and processes. CAES requires very large, pressurizable reservoirs to store the compressed air, dictated by the practical availability of aquifers, caverns, or salt domes, etc. for storing compressed air.
Faced with the dual tasks of carbon emission control and energy management, the energy industry can benefit tremendously from systems and methods providing an integrated solution for power management and carbon capture.